The citric acid cycle —also known as the Krebs cycle, Szent-Györgyi-Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. The Krebs cycle is used by organisms that respire (as opposed to organisms that ferment) to generate energy, either by anaerobic respiration or aerobic respiration. In addition, the cycle provides precursors of certain amino acids, as well as the reducing agent NADH, that are used in numerous other reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest components of metabolism. Even though it is branded as a 'cycle', it is not necessary for metabolites to follow only one specific route; at least three alternative segments of the citric acid cycle have been recognized.
Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis occurs mainly in the liver and, to a lesser extent, in the cortex of the kidneys. It is one of two primary mechanisms – the other being degradation of glycogen (glycogenolysis) – used by humans and many other animals to maintain blood sugar levels, avoiding low levels (hypoglycemia). In ruminants, because dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise, etc. In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise.
Biological carbon fixation or сarbon assimilation is the process by which inorganic carbon is converted to organic compounds by living organisms. The compounds are then used to store energy and as structure for other biomolecules. Carbon is primarily fixed through photosynthesis, but some organisms use a process called chemosynthesis in the absence of sunlight.
Oxaloacetic acid (also known as oxalacetic acid or OAA) is a crystalline organic compound with the chemical formula HO2CC(O)CH2CO2H. Oxaloacetic acid, in the form of its conjugate base oxaloacetate, is a metabolic intermediate in many processes that occur in animals. It takes part in gluconeogenesis, the urea cycle, the glyoxylate cycle, amino acid synthesis, fatty acid synthesis and the citric acid cycle.
In the mitochondrion, the matrix is the space within the inner membrane. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The mitochondrial matrix contains the mitochondrial DNA, ribosomes, soluble enzymes, small organic molecules, nucleotide cofactors, and inorganic ions.[1] The enzymes in the matrix facilitate reactions responsible for the production of ATP, such as the citric acid cycle, oxidative phosphorylation, oxidation of pyruvate, and the beta oxidation of fatty acids.
The enzyme citrate synthase E.C. 2.3.3.1 ] exists in nearly all living cells and stands as a pace-making enzyme in the first step of the citric acid cycle. Citrate synthase is localized within eukaryotic cells in the mitochondrial matrix, but is encoded by nuclear DNA rather than mitochondrial. It is synthesized using cytoplasmic ribosomes, then transported into the mitochondrial matrix.
Propionyl-CoA is a coenzyme A derivative of propionic acid. It is composed of a 24 total carbon chain and its production and metabolic fate depend on which organism it is present in. Several different pathways can lead to its production, such as through the catabolism of specific amino acids or the oxidation of odd-chain fatty acids. It later can be broken down by propionyl-CoA carboxylase or through the methylcitrate cycle. In different organisms, however, propionyl-CoA can be sequestered into controlled regions, to alleviate its potential toxicity through accumulation. Genetic deficiencies regarding the production and breakdown of propionyl-CoA also have great clinical and human significance.
The enzyme methylisocitrate lyase catalyzes the chemical reaction
The enzyme 2-methylcitrate dehydratase (EC 4.2.1.79) catalyzes the chemical reaction
The enzyme 2-methylisocitrate dehydratase (EC 4.2.1.99) catalyzes the chemical reaction
The enzyme homoaconitate hydratase (EC 4.2.1.36) catalyzes the chemical reaction
In enzymology, a 2-hydroxyglutarate synthase (EC 2.3.3.11) is an enzyme that catalyzes the chemical reaction
In enzymology, a citrate (Re)-synthase (EC 2.3.3.3) is an enzyme that catalyzes the chemical reaction
Decylcitrate synthase (EC 2.3.3.2) is an enzyme that catalyzes the chemical reaction in enzymology.
In enzymology, a decylhomocitrate synthase (EC 2.3.3.4) is an enzyme that catalyzes the chemical reaction
In enzymology, a homocitrate synthase (EC 2.3.3.14) is an enzyme that catalyzes the chemical reaction
In molecular biology, the citrate synthase family of proteins includes the enzymes citrate synthase EC 2.3.3.1, and the related enzymes 2-methylcitrate synthase EC 2.3.3.5 and ATP citrate lyase EC 2.3.3.8.
Methanogen homoaconitase (EC 4.2.1.114, methanogen HACN) is an enzyme with systematic name (R)-2-hydroxybutane-1,2,4-tricarboxylate hydro-lyase ((1R,2S)-1-hydroxybutane-1,2,4-tricarboxylate-forming). This enzyme catalyses the following chemical reaction
2-methylcitrate dehydratase (2-methyl-trans-aconitate forming) (EC 4.2.1.117) is an enzyme with systematic name (2S,3S)-2-hydroxybutane-1,2,3-tricarboxylate hydro-lyase (2-methyl-trans-aconitate forming). This enzyme catalyses the following chemical reaction
The methylcitrate cycle, or the MCC, is the mechanism by which propionyl-CoA is formed, generated by β-oxidation of odd-chain fatty acids, and broken down to its final products, succinate and pyruvate. The methylcitrate cycle is closely related to both the citric acid cycle and the glyoxylate cycle, in that they share substrates, enzymes and products. The methylcitrate cycle functions overall to detoxify bacteria of toxic propionyl-CoA, and plays an essential role in propionate metabolism in bacteria. Incomplete propionyl-CoA metabolism may lead to the buildup of toxic metabolites in bacteria, and thus the function of the methylcitrate cycle is an important biological process.
